This is happened to me too. I can watch any game just fine till they finished. But when I'm playing, no no no.
After reading the whole thread for a while, I think the problem will persist if you restart you connection without restarting you computer at all.

Jokingly, this is happened because Summer Sale doesn't get started.

Oh, and about you solution, I don't think that not really helping because I can download any games or updates from Steam just really fine. It's not about network issue. Its on Steam server.

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------TEMPORARY SOLUTION - Please report back, in case it solved the issue for you

Originally Posted by blash365

I'm getting Steam Validation Rejected or No Steam Logon often while I'm playing.
This error can be caused by account sharing. PLEASE DO NOT SHARE YOUR ACCOUNT. Account sharing is not permitted by the Steam Subscriber Agreement. You are responsible for the actions on your account.

If you are not sharing your account and you still receive this message, it indicates that Steam is having issues authenticating your ownership of Dota 2. It typically means that the Steam client is having difficulty connecting to the Steam servers. Please see this article to help identify issues with Steam connections, https://support.steampowered.com/kb_...1456-EUDN-2493 Several users have reported success when changing Steam to use TCP (as described in the linked article).

does this help any of you?

Originally Posted by tanikaze

Switching Steam to TCP rather than UDP might have fixed it for me. I'll report back if the problem comes back again. Weird that it just popped up with this patch, though.

so sad. i play and dc'ed and dc'ed. now i got my 4th abandon because of steam's server complication. i hope they can remove this abandonment points :C (sorry. now it's my fifth abandonment. that's it. i wont play until this is fixed.} :c